1. Field of the Invention
The present invention relates to halogen-free flame-retardant bitumen compositions for use in commercial and residential roof coverings such as roofing membranes and waterproofing underlayments. In particular, the present invention is directed to flame-retardant bitumen compositions with improved performance characteristics that achieve high fire retardancy with low loadings of non-halogenated compounds.
2. Description of the Related Art
Bitumen compositions have been used for decades on residential and commercial roofs as waterproof coverings. Bitumen has been used in shingles, roofing felts, roofing membranes, and similar products as a binder and waterproofing material. Bitumen is a combustible petroleum derivative and lacks any fire retardancy or self-extinguishing properties. Ignited roof coverings, containing untreated bitumen, propagate flames and spread the flames to other areas of the roof. Wind and high roof slopes often add to the intensity of roof fires fueled by untreated bitumen roof coverings.
Building code regulations in many jurisdictions require fire retardant roofing systems that comply with Class A or Class B fire resistance standards, such as those standards established by UL 790 or ASTM E-108, to eliminate or reduce the spread of flames in roof fires. Conventional systems for achieving fire retardancy in commercial roof coverings include surface treatment of the roofing material. Surface treatments include spreading stone chips or gravel over the roofing materials, or painting the surface of the roofing material with coatings that contain fire retardant compounds and/or asbestos. Although these surface treatment methods help provide the roof covering with a Class A or Class B fire rating, these treatment methods are time consuming and labor intensive and, therefore, increase the time required for roofing installation and the expense of the roofing material. A major disadvantage of these surface treatment methods is the weight of the final roofing system. The weight is often increased by gravel and stone chips that are usually applied at a concentration of 400 to 1000 pounds per 100 square feet of roof area (lb per 100 ft.sup.2). This concentration of gravel and stone chips is too heavy for many roof structures or "decks" such as roof decks made from plywood or gypsum. When gravel and stone chips are not applied to the roof covering by a flood coat of asphalt, the roofing system does not comply with most wind uplift resistance criteria such as that set forth by Factory Mutual's 4470/4450 Standard. Flame-retardant roof coatings that are painted onto the surface of the roofing material usually crack, peel, or wash away within three to five years from their application and have to be reapplied to maintain the fire rating of a roofing system.
Attempts have been made to incorporate flame-retardant additives directly into roofing materials in order to eliminate the need for gravel, stone chips, or flame-retardant coatings. Although a wide variety of flame-retardant additives are available, commercial demands require that flame-retardant additives be relatively inexpensive due to the competitive pricing structure of the roofing industry. Flame-retardant additives must also be water insoluble, thermally stable, and chemically stable in bitumen compositions. These characteristics are usually provided in bitumen roofing systems by using halogenated compounds or antimony oxide. Flame-retardant additives containing halogenated compounds, such as decabromodiphenyloxide, reduce combustibility of a bitumen roofing system by interfering with the gaseous phase of the burning reaction by releasing another halogen compound, such as bromine.
Compositions containing halogenated compounds or antimony oxide are suspected as possible contributors to the combustion toxicity of many plastic materials. Halogenated compounds and antimony oxide have been banned from use as fire retardant materials in many nations. Halogenated compounds and antimony oxide may present health hazards to the workers, who are involved with the manufacture of products made with these compounds. Additionally, halogenated compounds and antimony oxide release toxic air pollution when burned. In view of these hazards, many manufactures of roofing materials desire to eliminate halogenated compounds and antimony oxide from use in their products.
It has been recognized that antimony oxide can be replaced by several alternative compounds. An article by Stuart Wood, "Will Toxicity Concerns Doom Workhorse Flame-Retardant Systems?," Modern Plastics vol. 67 no.5 (May 1990) 40-44, discloses alumina trihydrate and/or zinc borate as an alternative flame-retardant compound for use in synthetic resin compositions such as plastics. Although alumina trihydrate does not require the use of halogen compounds in order to provide an effective flame-retardant compound, alumina trihydrate has a relatively low decomposition temperature and requires high concentrations or "loadings" so as to provide an effective fire resistance rating for a roofing material containing bitumen. Such high loadings of alumina trihydrate have an adverse affect on processing and physical properties of the resultant roofing material and significantly increase the expense of the finished product.
U.S. Pat. No. 5,055,135 to Grube et al. discloses a flame-retardant bituminous composition suitable for use as a roofing material. The composition contains 35 to 60 weight percent asphalt, 2 to 30 weight percent modifier, such as a styrene copolymer, and 35 to 50 weight percent of colemanite. Colemanite is an inorganic composition with fire retardant properties. Colemanite is a natural blend of hydrated oxides such as calcium borate. Other oxides, such as calcium oxide, silicon oxide, aluminum oxide, and magnesium oxide, can be included.
International Patent No. WO 91/02776 to Grube discloses a flame-retardant bitumen composition comprising 35 to 85 percent asphalt, 25 to 40 percent polymeric modifier, and colemanite. Colemanite is used as an inorganic fire retardant mixture in this composition. The bituminous mixture can be impregnated with a glass/polyester composite or fiberglass mats.
Many other presently available flame-retardant bitumen compositions contain large amounts or at least a 35 percent concentration of flame-retardant compounds. Other flame-retardant bitumen compositions include flame-retardant compounds that are partially water soluble. These partially water soluble flame-retardant compounds lose their effectiveness with continued exposure to weather. Other flame-retardant compositions utilize halogen compounds. The performance characteristics of roofing materials decrease, particularly upon aging, when the roofing materials contain a large concentration of inert fillers. Water soluble flame-retardant compounds in roofing material have particularly poor performance characteristics because these compounds are leached from the roofing material upon exposure to water. Partially or fully water soluble flame-retardant compounds, such as ammonium phosphates, can be encapsulated by water-resistant compounds including siloxanes, epoxies, melamine-formaldehydes, or similar polymers to prevent the flame-retardant compound from being leached from the roofing material. This encapsulation process, however, increases the production cost of the roofing material and can result in additional processing difficulties.